By-pass mechanism of transition to turbulence

The by-pass mechanism of transition for a wall-bounded shear layer is explained for the case when an infinite row of convecting vortices migrate over a boundary layer at a specific speed range. Such a mechanism is important for noisy flows over bluff bodies, flows inside turbo-machinery and flows over helicopter rotor blades. By solving the Navier-Stokes equation, it is shown that this by-pass transition is a consequence of vortices migrating at convection speeds that are significantly lower than the free-stream speed. This situation is commonly found in flows that are affected by the presence of periodic wakes. Whenever the speed of migrating vortices is in a certain critical range, there is a local instability of the underlying shear layer with a very high-growth rate as compared to the growth of pure Tollmien-Schlichting waves created by wall excitation. The above interpretation is supported by solving the linearized and full Navier-Stokes equation for disturbance quantities under the parallel flow approximation in two dimensions. Various ramifications of such a by-pass route of transition are discussed in this paper. (C) 2002 Academic Press.